Streptococcus pyogenes, also known as β-hemolytic group A streptococci (GAS) is a pathogen of humans which can cause mild infections such as pharyngitis and impetigo. Post infection autoimmune complications can occur, namely rheumatic fever and acute glomerulonephritis. GAS also causes severe acute diseases such as scarlet fever and streptococcal toxic shock syndrome (STSS). Severe GAS infections were a large problem in the U.S. and throughout the world at the beginning of this century. In the mid-forties, the number of cases and their severity decreased steadily for yet not completely understood reasons. However, more recently, a resurgence of serious diseases caused by GAS has been seen such that there may be 10-20,000 cases of STSS each year in the United States. As many as 50 to 60% of these patients will have necrotizing fascitis and myositis; 30 to 60% will die and as many as one-half of the survivors will have limbs amputated.
In 1986 and 1987 two reports described an outbreak of severe GAS infections localized in the Rocky Mountain area. These reports have been followed in the past few years by several others describing a disease with analogous clinical presentation. The symptoms described for this disease were very similar to those described for toxic shock syndrome (TSS), and in 1992 a committee of scientists gave to this clinical presentation the formal name of STSS, and set the criteria for its diagnosis. STSS is defined by the presence of the following:
1. hypotension and shock;
2. isolation of group A streptococci;
3. two or more of the following symptoms: fever 38.5° C. or higher, scarlet fever rash, vomiting and diarrhea, liver and renal dysfunction, adult respiratory distress syndrome, diffuse intravascular coagulation, necrotizing fascitis and/or myositis, bacteremia.
Streptococcal isolates from STSS patients are predominantly of M type 1 and 3, with M18 and nontypable organisms making up most of the reminder. The majority of M1, 3, 18, and nontypable organisms associated with STSS make pyrogenic exotoxin A (SPE-A, scarlet fever toxin A). In contrast, only 15% of general streptococcal isolates produce this toxin. Moreover, administration of SPE-A to a rabbit animal model and in two accidental human inoculations can reproduce the symptoms of STSS.
SPE-A is a single peptide of molecular weight equal to 25,787 daltons, whose coding sequence is carried on the temperate bacteriophage T12. speA, the gene for SPE-A, has been successfully cloned and expressed in Escherichia coli. SPE-A is a member of a large family of exotoxins produced by streptococci and staphylococci, referred to as pyrogenic toxins based upon their ability to induce fever and enhance host susceptibility up to 100,000 fold to endotoxin.
Recently these toxins have been referred to as superantigens because of their ability to induce massive proliferation of T lymphocytes, regardless of their antigenic specificity, and in a fashion dependent on the composition of the variable part of the β chain of the T cell receptor. These toxins also stimulate massive release of IFN-γ, IL-1, TNF-α and TNF-β. Other members of this family are streptococcal pyrogenic exotoxins type B and C, staphylococcal toxic shock syndrome toxin 1, staphylococcal enteroxtoxins A, B, Cn, D, E, G and H, and non-group A streptococcal pyrogenic exotoxins. These toxins have similar biochemical properties, biological activities and various degrees of sequence similarity.
The most severe manifestations of STSS are hypotension and shock, that lead to death. It is generally believed that leakage of fluid from the intravascular to the interstitial space is the final cause of hypotension, supported by the observation that fluid replacement therapy is successful in preventing shock in the rabbit model of STSS described above. It has been hypothesized that SPE-A may act in several ways on the host to induce this pathology.
SPE-A has been shown to block liver clearance of endotoxin of endogenous flora's origin, by compromising the activity of liver Kuppfer cells. This appears to cause a significant increase in circulating endotoxin, that through binding to lipopolysaccharide binding protein (LBP) and CD14 signaling leads to macrophage activation with subsequent release of TNF-α and other cytokines. Support for the role of endotoxin in the disease is given by the finding that the lethal effects of SPE-A can be at least partially neutralized by the administration to animals of polymyxin B or by the use of pathogen free rabbits.
Another modality of induction of shock could be the direct activity of the toxin on capillary endothelial cells. This hypothesis stems from the finding that the staphylococcal pyrogenic toxin TSST-1 binds directly to human umbilical cord vein cells and is cytotoxic to isolated porcine aortic endothelial cells.
Another of the toxin's modality of action includes its superantigenicity, in which the toxin interacts with and activates up to 50% of the host T lymphocytes. This massive T cell stimulation results in an abnormally high level of circulating cytokines TNF-β and IFN-(which have direct effects on macrophages to induce release of TNF-α and IL-1. These cytokines may also be induced directly from macrophages by SPE-A through MHC class II binding and signalling in the absence of T cells. The elevated levels of TNF-α and -β cause several effects typically found in Gram negative induced shock, among which is damage to endothelial cells and capillary leak. However, the administration to SPE-A treated rabbits of cyclosporin A, which blocks upregulation of IL-2 and T cell proliferation, did not protect the animals from shock, suggesting that additional mechanisms may be more important in causing capillary leak.
Thus, there is a need to localize sites on the SPE-A molecule responsible for different biological activities. There is a need to develop variants of SPE-A that have changes in biological activities such as toxicity and mitogenicity. There is a need to develop compositions useful in vaccines to prevent or ameliorate streptococcal toxic shock syndrome. There is also a need to develop therapeutic agents useful in the treatment of streptococcal toxic shock syndrome and other diseases.